The Wilderness Society says it has been pleasantly surprised by a senator's decision to oppose a proposal to roll back Queensland's Wild Rivers legislation.

The Federal Opposition's bill lost the support of Family First Senator Steve Fielding, whose vote was crucial to its success.

Some Cape York Indigenous advocates say Senator Fielding had voted for an earlier version of the bill and they are upset he has changed his mind.

Dr Tim Seelig from the Wilderness Society says he is also surprised by Senator Fielding's announcement.

"He wasn't expected to put his neck out and put the environment over mining but that's very, very clearly what he's done," he said.

"It was very obvious from this speech that he's understood how Wild Rivers actually works, he's understood what's wrong with the Abbott bill - that it has many, many flaws - and that it's unworkable and that he wouldn't support it."

Dr Seelig says he cannot see the bill being passed.

"There's only a few sitting days left in the Senate before it changes," he said.

"Our assessment is that it is effectively dead in the water if it goes to a vote before the end of June and it's the same bill.

"Unless people make some radical shifts in their current positions, then the vote would be defeated."